This invention pertains to the prevention of corrosion of ferrous metals of aqueous potassium carbonate solutions in acid gas-treating plants.
Aqueous potassium carbonate solutions are used for the removal of carbon dioxide, hydrogen sulfide and other acidic gaseous constituents from natural gas, flue gas, synthesis gas, and the like. In this gas-treating process the aqueous potassium carbonate is continuously cycled from an absorber, in which acidic gases are taken up, to a stripper in which the acidic gases are expelled by modifying the overhead pressure and the solution temperature. The potassium carbonate content of these solutions can vary within a range of about 20 to about 40 percent, depending upon the individual installation, and may contain monoethanolamine, diethanolamine, amine borates, and the like to assist gaseous absorption. In all of these compositions however, the absorbing solutions are very corrosive to ferrous metals with which they come into contact.
Numerous investigators have studied the corrosion of aqueous potassium carbonate solutions and the incorporation of additives in them for corrosion prevention. For example Bienstock and Field reported in Corrosion, Vol. 17, page 337t (1961) that higher concentrations of potassium carbonate were more corrosive than lower concentrations and that sparging with carbon dioxide greatly increased this corrosion. Analysis of the solutions showed that carbon dioxide caused conversion of 15 to 20 percent of the carbonate to bicarbonate, thus suggesting that the bicarbonate is the more corrosive species. If the sparging gas contained hydrogen sulfide, corrosion was less even if carbon dioxide were present but in commercial plant usage, operating problems still resulted from corrosion. Bienstock and Field subsequently reported in Corrosion, Vol. 17, page 571t (1961) that 0.2 percent of sodium chromate or sodium metavanadate prevented most of the attack of ferrous metals although some pitting was noted in crevices with the chromate-inhibited solutions.
Negra and McCloskey disclosed in U.S. Pat. No. 3,087,778 that trivalent compounds of arsenic, antimony, bismuth and phosphorus acted as inhibitors for both liquid and vapor corrosive attack.
Banks, in Material Protection, Vol. 6, page 37 (1967) studied the corrosivity of used solutions from gas absorption plant installations and laboratory-prepared solutions containing concentrations of potassium carbonate and potassium bicarbonate typical of those used in service. His polarization studies indicated that metavanadate salts passivate mild steel only if the bicarbonate level was low. Once passivated, however, the steel would remain even so if the carbonate were partly converted to bicarbonate by carbon dioxide. In tests simulating plant conditions, corrosion was greater under impingement conditions such as at elbows than when film disruptive conditions were absent. Under such conditions even stainless steel alloys may also be attacked.
Eickmeyer in British Pat. No. 1,142,317 revealed that although 0.1 to 0.3 percent of sodium metavanadate effectively inhibits attack of steel in laboratory experiments, such concentrations were inadequate in actual plant use. He attributed this phenomenon to a more favorable solution volume to exposed steel surface in laboratory experiments which in effect results in more inhibitor ions being available for metal protection. He teaches that it is necessary to employ an oxidant in conjunction with vanadium ions in order to keep them active as corrosion inhibitors.